This invention relates generally to cathode ray tubes (CRTs) and display devices and is particularly directed to improving the sealed coupling between the faceplate and the funnel of a CRT or between other CRT components, e.g., between a shadow mask supporting structure and the CRT's faceplate.
In the final assembly of the envelope of a CRT, a frit material, typically a suspension of thermal setting sealing glass paste, is deposited upon one of the abutting surfaces of the faceplate or funnel, generally the forward edge of the funnel, prior to positioning of the faceplate and funnel in intimate contact. The faceplate and funnel combination is then heated to the melting temperature of the sealing glass frit which then flows so as to cover and join abutting surfaces of the faceplate and envelope. The sealing glass frit is heated to a temperature so as to effect its devitrification whereupon at least a part of the frit is converted, or devitrified, to a crystalline phase, or a rigid crystalline skeleton. The crystalline phase of the frit is characterized by a glassy matrix wherein the thermal and other material characteristics such as viscosity and coefficient of expansion differ from those of the original sealing glass frit and are substantially determined by the crystalline phase. The melting point temperature of the devitrified sealing glass frit is lower than the fiber softening point temperature of the glass components to allow for a bakeout operation of the CRT whereupon organic vapors are volatilized or vaporized and removed from the CRT.
The cohesive nature and high surface tension of the sealing glass frit results in the formation of "re-entrancies", giving rise to stress concentrations in the faceplate-funnel seal resulting in potential weakening of this joint. Breakdown of this seal results in failure of the CRT and requires its replacement. When this occurs during CRT manufacture the tube is scrapped and an attempt is made to salvage any usable components and materials within the CRT.
The occurrence of such re-entrant seals in the manufacture of cathode ray tube envelopes is a serious problem causing a significant reduction in tube yields (percentage of acceptable tubes) in the factory. Defects introduced in the final stages of tube fabrication are especially costly, as the value of the assembly-in-process is approaching then its maxlmum. The problem of re-entrant seals has eluded solution by practitioners for decades in spite of a universal recognition of its huge cost to tube manufacturers.
The recent development of CRTs having a substantially flat faceplate and incorporating a shadow mask of the tensioned foil type has placed increasing demands upon the seal between the faceplate and funnel. For example, flat faceplate CRTs exhibit greater stress in the sealing area than conventional CRTs incorporating a curved faceplate having a rearward flange to facilitate its coupling to a funnel. In order to minimize stresses in the seal area accompanying bulb flexure under thermal processing and evacuation, these CRTs exhibit wider seal areas at the funnel. In addition, the panel is made to overhang the funnel in order to minimize the possibility of severe re-entrancies in the funnel-to-panel seal interface which can occur if the funnel exterior and panel edge are nearly "line-to-line." This latter situation can occur in bulb assembly fixturing as the result of glass dimensional tolerances. The consequence of these accommodations is to increase the overall footprint size of the panel and the weight of the tube. In addition, the foil mask requires a mounting structure which, under the load of the tensed mask, induces stresses at its bonded interface with the bulb envelope.
The response of the various CRT components to the stresses described above includes elastic deformations due to vacuum and tension mask loads as well as thermal deformations occurring during CRT processing. The thermal responses include "differential" deformations accompanying thermal gradients during CRT processlng as well as responses of joined CRT components havlng different coefficients of thermal expansion. The internal load patterns whlch accompany the deformations result in load paths which pass through the component junctures.
The internal load pattern within the CRT gives rise to internal stresses, with the load path through adjacent component junctures producing stresses in the sealing glass frit which forms the structural joint. If the joint exhibits a re-entrant geometry, stress concentrations due to this geometry will occur in the joint, increasing the possibility of structural failure. This is of particular concern in the case of flat panel CRTs which undergo higher panel-to-funnel interface stresses than conventional domed face, skirted panel CRTs. The sealing glass frit is highly cohesive and does not readily wet glass. Consequently, in the prior assembly of CRT components, the sealing glass frit used in forming the joint tends to roll back upon itself and re-entrancies are formed.
The present invention addresses the aforementioned problems of the prior art by providing improved sealing for a CRT faceplate mounted to the forward edge portion of the CRT's funnel. The present invention, which is applicable to CRTs having a flat faceplate as well as to those incorporating a curved faceplate, compensates for the cohesive nature of the sealing glass frit used to couple the CRT's faceplate and funnel and minimizes stress concentrations in the connections between various structures in the CRT.
Related problems arise in other structural bonds including glass-to-ceramic, glass-to-metal, metal-to ceramic, or related composition in the manufacture of CRTs and these problems, too, are addressed by this invention.